The invention involves a method for producing a low aluminum content phosphoric acid from high aluminum-content phosphate matrix.
The invention also involves controlling the fluoride content of steady-state phosphoric acid produced from unbeneficiated high aluminum-content matrix (especially matrix with a high clay content) in order to prevent premature formation of the novel layered aluminum fluorophosphate of composition AlFHPO.sub.4.2H.sub.2 O which is described in the commonly owned, copending application of Chemtob and Beer which was filed on the same day as the subject application and is hereby incorporated herein.
The invention can also be used to prevent or greatly reduce formation of the aluminum fluorophosphate by controlling the fluorine-content of the phosphoric acid at a sufficiently low level (e.g., usually below 1 weight %).
The present invention also involves a means of converting high alumina content phosphate matrix into a relatively low aluminum content phosphoric acid without the usual beneficiation by floatation. This process permits conversion to phosphoric acid (and aluminum phosphate) of a much greater proportion of the phosphate values in the matrix. For example, in the usual beneficiation of phosphate matrix, by the dihydrate or hemihydrate routes, only about fifty percent of the phosphate values in the matrix are recovered in the beneficiated product. In contrast, about eighty percent of the phosphate values in the matrix can be recovered by the present process.
Phosphate reserves are sedimentary deposits formed by reprecipitation of dissolved phosphate from prehistoric seas. For example, a typical North Florida phosphate ore consists primarily of fluorapatite (a phosphate-containing mineral), quartz sand, and clay minerals. This ore body is called the phosphate matrix.
In current mining practice, the matrix is excavated by draglines, slurried with water at high pressure (e.g. about 200 pounds per square inch) and pumped through miles of pipeline to the beneficiation plants where sand and clays are removed from the fluorapatite by floatation processes, producing the so-called beneficiated phosphate rock.
Current commercial processes call for the usage of either beneficiated or high quality phosphate rock and sulfuric acid as raw materials to produce either hemihydrate or dihydrate phosphoric acid. In most cases where beneficiation operations are required, losses of about 40% P.sub.2 O.sub.5 values in the matrix occur in the form of slimes and tailings. The slimes are discharged to storage ponds as a dilute slurry containing about 5% of fine particulate minerals, which take years to settle. For every acre-ft of matrix mined, about 1.5 acre-ft of slime is produced as a result of beneficiation. Accordingly, rock beneficiation creates an environmental concern in addition to the large loss of P.sub.2 O.sub.5 values.
In U.S. Pat. No. 3,792,151 to Case, phosphoric acid is produced from low BPL (bone phosphate of lime or tricalcium phosphate) phosphate rock having about 1.5% fluorine by a process comprising reacting the phosphate rock with an equilibrated phosphoric acid having a P.sub.2 O.sub.5 concentration between about 20 to 50% in an attack stage at a temperature below about 180.degree. F., said equilibrated acid being essentially saturated with respect to the fluorine component of said rock at the temperature of said attack stage; said temperature and the time of reaction serving to dissolve at least about 90 percent of the tricalcium phosphate in the rock to produce a monocalcium phosphate-phosphoric acid-water solution up to about 90 percent saturated with monocalcium phosphate and containing insoluble material and a soluble fluorine content of from about 1 to 3 percent, the weight ratio of P.sub.2 O.sub.5 in the acid to P.sub.2 O.sub.5 in the rock feed being sufficient to dissolve tricalcium phosphate values in the rock and provide the desired solution and at least about 7:1, separating the insoluble material from the solution to produce a solution of monocalcium phosphate-phosphoric acid-water, said solution having a fluorine content of from 1 to 3 percent, reacting sulfuric acid with said solution to produce phosphoric acid and precipitate calcium sulfate, the sulfuric acid being added in an amount essentially stoichiometric with respect to the monocalcium phosphate in the solution, separating the calcium sulfate from the phosphoric acid solution, removing a portion of the phosphoric acid as product, and recycling the remaining phosphoric acid solution to the attack stage to provide said equilibrated acid and removing a portion of the phosphoric acid as product. There is no disclosure in the Case patent of a process for removing alumina from the product acid by forming an aluminum fluorophosphate, nor of fluorine control. Phosphoric acid produced by the process of this invention is not an equilibrated P.sub.2 O.sub.5 because of the removal of the aluminum and fluorine in the precipitation of the aluminum fluorophosphate. In the invention the phosphoric used to dissolve the matrix is not equilibrated because of the controlled removal of fluorine therefrom, or the low aluminum and low fluorine content phosphoric acid product of aging is used to dissolve the tricalcium phosphate in the matrix; thereby controlling the fluoride content of the crystallization (of calcium sulfate) step such that the aluminum fluorophosphate does not form until after the gypsum separation. The invention also involves controlling the fluoride content by other means, such as volatilization and addition of sodium or potassium compounds.
In the manufacture of synthetic cryolite, an aluminum fluoro phosphate AlF.sub.2 H.sub.2 PO.sub.4 is reported in U.S. Pat. No. 3,672,189 to Betts. This composition is different from that produced in the present process, in that it is relatively higher in HF than in the novel AlFHPO.sub.4 of the present invention. Also, the production of the Betts compound would not lower the aluminum content of phosphoric acid to as great an extent as does the production of AlFHPO.sub.4.2H.sub.2 O as disclosed hereinafter. The process steps involved in the manufacture of synthetic cryolite are quite different from the process for manufacture of the novel aluminum fluorophosphate of the present invention.
Aluminum fluorophosphate of composition Al(HPO.sub.4) F.2H.sub.2 O is reported in the July 1980 Russian Journal of Inorganic Chemistry 25(7) 1980; however, this compound is reported as being formed by a process involving adding aluminum sulphate solution to a mixture of phosphoric acid and ammonium fluoride. The reagents used were "pure" or "highly pure" grades. No work is reported in the Russian Journal article of a process whereby AlFHPO.sub.4.2H.sub.2 O is prepared from impure phosphoric acid (e.g., green or black acid or from a high alumina content phosphoric acid produced from unbeneficiated matrix).
J. W. Akitt, N. N. Greenwood, and G. D. Lester, "Nuclear Magnetic Resonance and Raman Studies of the Aluminum Complexes formed in Aqueous Solutions of Aluminum Salts Containing Phosphoric Acid and Fluoride Ions," J. Chemical Society (A), 1971, mention the existence of a liquid phase of the complex AlF.sub.2 H.sub.2 PO.sub.4.
Single stage and continuous matrix processes are described by P. C. Good, T. N. Goff and J. C. White in Report of Investigations 8339, titled "Acidulation of Florida Phosphate Matrix in a Single-Tank Reaction" and Report of Investigations 8326, titled "Continuous-Circut Preparation of Phosphoric Acid From Florida Phosphate Matrix" (published by the U.S. Department of the Interior, Bureau of Mines). These processes are similar to those described herein; however, no mention is made therein of solids removal prior to formation of calcium sulfate or how to successfully produce low aluminum-content acid where the process is continuous and aluminum and fluorine build-up continuously in the recycle acid.
Methods of removing fluorine from wet process phosphoric acid are summarized at pages 696-701 of Chapter 8, Volume 1, Part II of Phosphoric Acid, edited by A. V. Slack, 1968, Marcel Dekker, Inc., New York. Although all of these methods can be used in the present invention, it is preferred to control the fluoride content by precipitation of insoluble fluoride compounds, especially by adding a sodium or potassium compound, or both the acid being treated.
Herein percentages are by weight unless otherwise specified.